Synthesis of a β-(1→3)-D-rhamnotetraose by a one-pot, multiple radical fragmentation

Article abstract of DOI:10.1021/ol061706m

(Chemical Equation Presented) A naturally occurring β-(1→3)-D- rhamnotetraose has been constructed under conditions of sequential β-selective mannosylation controlled by the 4,6-O-[1-cyano-2-(2-iodophenyl) -ethylidene] protecting group. The route is conci

Full text of DOI:10.1021/ol061706m

ORGANIC
LETTERS
2006
Vol. 8, No. 19
4327-4330
Synthesis of a
-(1 3)- -Rhamnotetraose by a
One-Pot, Multiple Radical Fragmentation
â
f
D
David Crich* and Albert A. Bowers
Department of Chemistry, UniVersity of Illinois at Chicago, 845 West Taylor Street,
Chicago, Illinois 60607-7061
dcrich@uic.edu
Received July 11, 2006
ABSTRACT
A naturally occurring
â
-(1f
3)-D-rhamnotetraose has been constructed under conditions of sequential
â
-selective mannosylation controlled by
the 4,6-O-[1-cyano-2-(2-iodophenyl)-ethylidene] protecting group. The route is concise, proceeding through a late-stage radical deoxygenation
that successfully uncovers all four deoxy subunits at once.
6-Deoxy-D-mannose, called D-rhamnose, has been found
exclusively in antigenic lipopolysaccharides (LPSs) associ-
ated with the cell walls of microorganisms.1 Some examples
include Xanthomonas campestris,2 Pseudomonas cepacia,1c
P. syringae pV. Morspurunorum,3 P. aeruginosa IID 1008,4
P. maltophilia 555,5 Myxobacterium 402,6 and Escherichia
hermanii.7 To date, this novel subunit has not been encoun-
tered in humans, plants, or animals. Given the rise in
antimicrobial resistance, enzymes involved in the biosyn-
thesis of the D-rhamnopyranosides would make promising
targets for potentially xenobiotic antiinfectives.
E. hermanii is a member of the family of enterobacteri-
aceae, related to E. coli. It has been isolated from human
wounds and sputum and has demonstrated pathogenicity
against humans in vivo.8 E. hermanii produces a â-lactamase
and exhibits a distinctive antibiotic resistance penicillin,
ampicillin, and carbenicillin.9 Degradation studies have
resulted in characterization of a repeat (1f3)-â-D-rhamnan
from the cell walls of E. hermanii strain ATCC 33651.10
The high content of the difficult â-D-rhamnosyl linkage
combined with its potential medicinal relevance make this
LPS O-chain constituent an appropriate candidate for de-
velopment of methods aimed at synthesis of the â-D-
rhamnopyranosides.
(1) (a) Hirooka, M.; Yoshimura, A.; Saito, I.; Ikawa, F.; Uemoto, Y.;
Koto, S.; Takabatake, A.; Taniguchi, A.; Shinoda, Y.; Morinaga, A. Bull.
Chem. Soc. Jpn. 2003, 76, 1409. (b) Spitali, M.; Smith, A. R. W. J.
Phytopath. 2000, 148, 563. (c) Knirel, Y. A.; Shashkov, A. S.; Senchenkova,
S. Y. N.; Ajiki, Y.; Fukuoka, S. Carbohydr. Res. 2002, 337, 1589. (d)
Cerantola, S.; Montrozier, H. Eur. J. Biochem. 1997, 246, 360. (e) Winn,
A. M.; Wilkinson, S. G. Carbohydr. Res. 1996, 294, 109. (f) Senchenkova,
S. N.; Shashkov, A. S.; Kecskes, M. L.; Ahohuendo, B. C.; Knirel, Y. A.;
Rudolph, K. Carbohydr. Res. 2000, 329, 831. (g) Vinogradov, E. V.;
Campos-Portuguez, S.; Yokota, A.; Mayer, H. Carbohydr. Res. 1994, 261,
103. (h) Vinogradov, E. V.; Shashkov, A. S.; Knirel, Y. A.; Zdorovenko,
G. M.; Solyanik, L. P.; Gubanova, N. Y.; Yakovleva, L. M. Carbohydr.
Res. 1991, 212, 307. (i) Molinaro, A.; Silipo, A.; Lanzetta, R.; Newman,
M.-A.; Dow, J. M.; Parrilli, M. Carbohydr. Res. 2003, 338, 277. (j) Beynon,
L. M.; Bundle, D. R.; Perry, M. B. Can. J. Chem. 1990, 68, 1456.
(2) Hickman, J.; Ashwell, G. J. Biol. Chem. 1966, 241, 1424.
(3) Smith, A. R. W.; Zamze, S. E.; Munro, S. M.; Carter, K. J.; Hignett,
R. C. Eur. J. Biochem. 1985, 149, 73.
The stereoselective synthesis of the 1,2-cis-equatorial
glycosidic bond as is found in both the â-mannosides and
the â-rhamnosides is of perennial difficulty in carbohydrate
chemistry.11 Without the possibility of invoking neighboring
group participation, the synthesis of such a linkage is
(4) Yokota, S.; Kaya, S.; Sawada, S.; Kawamura, T.; Araki, Y.; Ito, E.
Eur. J. Biochem. 1987, 167, 203.
(5) Di Fabio, J. L.; Perry, M. B.; Bundle, D. R. Biochem. Cell Biol.
1987, 65, 968.
(6) Morrison, I. M.; Young, R.; Perry, M. B.; Adams, G. A. Can. J.
Chem. 1967, 45, 1987.
(8) Dahl, K. M.; Barry, J.; DeBiasi, R. L. Clin. Infect. Dis. 2002, 35,
96.
(9) (a) Fitoussi, F.; Arlet, G.; Grimont, P. A. D. J. Antimicrob. Chemother.
1995, 36, 537. (b) Chaudhury, A.; Nath, G.; Tikoo, A.; Sanyal, S. C. J.
Diarr. Dis. Res. 1999, 17, 85.
(7) Perry, M. B.; Bundle, D. R. Infect. Immun. 1990, 58, 1391.
(10) Perry, M. B.; Richards, J. C. Carbohydr. Res. 1990, 205, 371.
10.1021/ol061706m CCC: $33.50
© 2006 American Chemical Society
Published on Web 08/17/2006

rendered somewhat more sensitive than that of the trans-
glycosidic bond. Captivated by this challenge, our group has
found considerable success in employing the torsionally and
electronically disarming 4,6-O-benzylidene protecting group.12
In our researches, this group has been used to synthesize a
variety of â-D-mannopyranosides, including the (1f2)-,
(1f4)-, and alternating (1f3)-, (1f4)-mannans.13 However,
even with this technology in hand, the challenge of the cis-
glycosidic bond is considerably magnified in the biologically
important rhamnopyranosides, which lack the functional arm
for incorporation of a benzylidene-type directing effect.
In general, strategies for synthesis of polysaccharides
containing deoxy-sugars proceed via prior synthesis of an
appropriately protected deoxy subunit, followed by extensive
optimization of conditions for stereoselective glycosidation;
recent efforts in this vein have been frustrated by low
selectivities.14 However, the reliability of the benzylidene-
mediated mannosylations combined with their close structural
relationship to the â-rhamnosides suggest inverting such a
paradigm: synthesis of â-mannosyl linkages followed by a
deoxygenation to provide the otherwise demanding subunits.
This strategy is particularly attractive in the case of the â-D-
rhamnopyranosides, where the starting material, D-mannose,
is easily available in bulk. Thus, we have recently developed
a protecting group that readily combines the stereoselectivity
of a benzylidene acetal with a latent radical fragmentation
pathway, providing a high-yielding deoxygenation in the last
stage of oligosaccharide synthesis.15 Herein, we demonstrate
the broadest capabilities of this method to date, with a concise
total synthesis of a tetrasaccharide fragment from the (1f3)-
rhamnan of E. hermanii (ATCC 33651) via a one-pot
quadruple radical fragmentation. To the best of our knowl-
edge, this is the first synthesis of a â-rhamnan (of either the
D- or L-modification) and an unique example of such a
multiple radical deoxygenation.16
Benzylidene-protected hexopyranosides are known to
undergo deoxygenation at C-6 via the NBS-mediated
Hanessian-Hullar reaction.17 However, the initiation step
of this reaction, radical abstraction of the benzylidene proton,
has proven indiscriminate in consort with the standard host
of nonparticipating protecting groups necessary for oligo-
saccharide synthesis.18 Similar incompatibilities are observed
with Roberts’ thiol-catalyzed benzylidene fragmentation.19
Whereas the Hanessian-Hullar reaction likely occurs via a
radical/polar crossover mechanism, Roberts’ sequence pro-
ceeds via a purely radical mechanism.20 This mechanism
favors fragmentation to a primary radical at C-6 due to a
conformationally less-strained transition state arising from
planarization at the incipient C-6 radical.21
To avoid the problematic hydrogen atom abstraction step,
we introduced the 4,6-O-[R-(2-(2-iodophenyl)-ethylthio-
carbonyl)-benzylidene] group.22 This group enabled the
synthesis of the tetrasaccharide subunit from E. hermanii
(ATCC 33650 and 33652).23 However, the limited functional
group compatibility of a key transesterification required to
introduce the group minimized the overall scope. In subse-
quent work, we have identified a second-generation 4,6-O-
[1-cyano-2-(2-iodophenyl)-ethylidene] acetal as a surrogate
for the benzylidene fragmentation that is easily prepared, is
easily installed, and is orthogonal to many protecting group
manipulations.
The mechanism for the cyano group transfer/fragmentation
(Scheme 1) is based upon chemistry first articulated by
Scheme 1. Radical Fragmentation Mechanism
(11) (a) Barresi, F.; Hindsgaul, O. In Modern Methods in Carbohydrate
Synthesis; Khan, S. H., O’Neill, R. A., Eds.; Harwood Academic Publishers:
Amsterdam, 1996; pp 251. (b) Demchenko, A. V. Synlett 2003, 1225. (c)
Pozsgay, V. In Carbohydrates in Chemistry and Biology; Ernst, B., Hart,
G. W., Sinay, P., Eds.; Wiley-VCH: Weinheim, Germany, 2000; Vol. 1,
pp 319. (d) Gridley, J. J.; Osborn, H. M. I. J. Chem. Soc., Perkin Trans. 1
2000, 1471.
(12) (a) Crich, D.; Li, H. J. Org. Chem. 2002, 67, 4640. (b) Crich, D.;
Dai, Z. Tetrahedron 1999, 55, 1569. (c) Crich, D.; Barba, G. R. Tetrahedron
Lett. 1998, 39, 9339. (d) Crich, D.; de la Mora, M. A.; Cruz, R. Tetrahedron
2002, 58, 35. (e) Crich, D.; Banerjee, A. Org. Lett. 2005, 7, 1395. (f) Crich,
D.; Dudkin, V. J. Am. Chem. Soc. 2002, 124, 2263. (g) Dudkin, V. Y.;
Crich, D. Tetrahedron Lett. 2003, 44, 1787. (h) Dudkin, V. Y.; Miller, J.
S.; Danishefsky, S. J. J. Am. Chem. Soc. 2004, 126, 736. (i) Miller, J. S.;
Dudkin, V. Y.; Lyon, G. J.; Muir, T. W.; Danishefsky, S. J. Angew. Chem.,
Int. Ed. 2003, 42, 431. (j) Nicolaou, K. C.; Mitchell, H. J.; Rodriguez, R.
M.; Fylaktakidou, K. C.; Suzuki, H.; Conley, S. R. Chem.-Eur. J. 2000,
6, 3149. (k) Kim, K. S.; Kang, S. S.; Seo, Y. S.; Kim, H. J.; Lee, Y. J.;
Jeong, K.-S. Synlett 2003, 1311. (l) Wu, X.; Schmidt, R. R. J. Org. Chem.
2004, 69, 1853. (m) Kwon, Y. T.; Lee, Y. J.; Lee, K.; Kim, K. S. Org.
Lett. 2004, 6, 3901. (n) Tanaka, S.-I.; Takashina, M.; Tokimoto, H.;
Fujimoto, Y.; Tanaka, K.; Fukase, K. Synlett 2005, 2325.
Beckwith and later expanded by Rychnovsky.24 As is
frequently the case with radical reactions propagated by tin
(13) (a) Crich, D.; Banerjee, A.; Yao, Q. J. Am. Chem. Soc. 2004, 126,
14930. (b) Crich, D.; Li, W.; Li, H. J. Am. Chem. Soc. 2004, 126, 15081.
(c) Crich, D.; Li, H.; Yao, Q.; Wink, D. J.; Sommer, R. D.; Rheingold, A.
L. J. Am. Chem. Soc. 2001, 123, 5826.
(14) Bedini, E.; Carabellese, A.; Barone, G.; Parrilli, M. J. Org. Chem.
2005, 70, 8064.
(15) Crich, D.; Bowers, A. A. J. Org. Chem. 2006, 71, 3452.
(16) For a previous example of a multiple radical fragmentation in
oligosaccharide synthesis, see: Crich, D.; Hermann, F. Tetrahedron Lett.
1993, 34, 3385.
(17) (a) Hanessian, S.; Plessas, N. R. J. Org. Chem. 1969, 34, 1035. (b)
Hanessian, S.; Plessas, N. R. J. Org. Chem. 1969, 34, 1045. (c) Hanessian,
S.; Plessas, N. R. J. Org. Chem. 1969, 34, 1053. (d) Chana, J. S.; Collins,
P. M.; Farnia, F.; Peacock, D. J. J. Chem. Soc., Chem. Commun. 1988, 94.
(e) Binkley, R. W.; Goewey, G. S.; Johnston, J. C. J. Org. Chem. 1984,
49, 992. (f) Hanessian, S. Org. Synth. 1987, 65, 243. (g) Hullar, T. L.;
Siskin, S. B. J. Org. Chem. 1970, 35, 225.
(18) Liotta, L. J.; Dombi, K. L.; Kelley, S. A.; Targontsidis, S.; Morin,
A. M. Tetrahedron Lett. 1997, 38, 7833.
4328
Org. Lett., Vol. 8, No. 19, 2006

Table 1. Preparation of Fragmentation Precursor 18 by Iterative Glycosidationa
a Reagents: (1) Ph2SO (1.5 equiv), TTBP (3.0 equiv), Tf2O (1.7 equiv), CH2Cl2, -70/-20/-70 °C. (2) DDQ, CH2Cl2/H2O (17:1).
hydrides, several competing reactions are possible, including
premature reduction of radicals 2-4, making the rapid rate
of cyano group migration essential for our synthesis. The
challenge of synthesizing a polymeric rhamnan by this
methodology can be seen as one of minimizing a possible
three different byproducts per monomer subunit or, for a
tetrasaccharide, promoting one product in 34. With this in
mind, we commenced our synthesis.
The synthesis of the (1f3)-tetrasaccharide required prepa-
ration of only one suitably protected monomer, 11. This was
achieved from diol 9, which was prepared from 4,6-O-
benzylidene-protected thiomannoside 825 in 72% yield over
four steps using standard reactions (Scheme 2). The 4,6-O-
deprotection was achieved prior to chromatographic purifica-
tion of the newly synthesized oligomer. Thus, a single
purification provided the acceptor for each subsequent step
in the elaboration of the growing chain. All couplings resulted
in high yields and high â-selectivities, as is consistent with
a benzylidene-type directing effect (Table 1). The â-anomers
could be readily assigned by their characteristic H-5 mul-
tiplets in the 1H NMR spectrum, with the reducing end H-5
resonance at δ ∼3.3 and subsequent residues further upfield
still at δ 2.6-2.9.27
The fully protected tetrasaccharide was subjected to
conditions of radical fragmentation in refluxing xylenes. In
the development of the protecting group, it was found that
the higher boiling point of xylenes favored the fragmentation
pathway over the reduction of the benzylidene radical 4.
Scheme 2. Synthesis of Donor 11
(19) (a) Roberts, B. P.; Smits, T. M. Tetrahedron Lett. 2001, 42, 3663.
(b) Dang, H.-S.; Roberts, B. P.; Sekhon, J.; Smits, T. M. Org. Biomol.
Chem. 2003, 1, 1330. (c) Fielding, A. J.; Franchi, P.; Roberts, B. P.; Smits,
T. M. J. Chem. Soc., Perkin Trans. 2 2002, 155. (d) Cai, Y.; Dang, H.-S.;
Roberts, B. P. J. Chem. Soc., Perkin Trans. 1 2002, 2449. (e) Jeppesen, L.
M.; Lundt, I.; Pedersen, C. Acta Chem. Scand. 1973, 27, 3579.
(20) (a) McNulty, J.; Wilson, J.; Rochon, A. C. J. Org. Chem. 2004, 69,
563. (b) Crich, D.; Bowers, A. A.; Yao, Q. Carbohydr. Res. 2006, 341,
1748.
(21) Roberts, B. P.; Dang, H.-S.; Cai, Y. J. Chem. Soc., Perkin Trans.
1 2002, 2449.
(22) Crich, D.; Yao, Q. Org. Lett. 2003, 5, 2189.
(23) Crich, D.; Yao, Q. J. Am. Chem. Soc. 2004, 126, 8232.
(24) (a) Beckwith, A. L. J.; Easton, C. J. J. Am. Chem. Soc. 1981, 103,
615. (b) Rychnovsky, S. D.; Swenson, S. S. Tetrahedron 1997, 53, 16489.
(25) Crich, D.; Li, W.; Li, H. J. Am. Chem. Soc. 2004, 126, 15081.
(26) Utimoto, K.; Wakabayashi, Y.; Horiie, T.; Inoue, M.; Shishiyama,
Y.; Obayashi, M.; Nozaki, H. Tetrahedron 1983, 39, 967.
(27) That it was the residues of the nonreducing end with upfield
resonances was confirmed by NOE experiments on the â-dimer 13. In the
NOE spectrum, a clear correlation was observed between the cyclohexyl
proton and the reducing end anomeric proton, which was further correlated
to the downfield H-5 signal. The upfield H-5 multiplet correlated with the
remaining anomeric peak.
[1-cyano-2-(2-iodophenyl)-ethylidene] acetal was cleanly
introduced as a single diastereomer via Lewis acid promoted
transcyanation chemistry developed by Utimoto and co-
workers.26 Monomer 11 was then employed in the sequential,
linear synthesis of a â-mannotetraose. After each coupling,
Org. Lett., Vol. 8, No. 19, 2006
4329

Scheme 3. Radical Fragmentation and Deprotection of Tetrasaccharide 18
Adapting the conditions from the developmental work, a 4
hydroxide and hydrogen proceeded to give the tetrasaccharide
h addition of tin hydride to a 0.0015 M solution of substrate
in xylenes at reflux, followed by NaBH4 reduction to
facilitate removal of the tin residues, and then saponification
allowed initial separation of the desired product cleanly on
silica gel from traces of byproducts arising from incomplete
fragmentation. The pure tetraol, 20, was isolated in 22% yield
from 18. Subsequent global deprotection with palladium
21 in 90% yield (Scheme 3).
1
In both the H and 13C NMR spectra of the synthetic
tetrasaccharide, resonances from the two end units can be
distinguished from the compounded peaks of the internal
subunits. Despite these differences, there is excellent agree-
ment between shifts of the internal residues of the synthetic
polymer and those of the natural tetrasaccharide, as is
illustrated in Table 2.
In conclusion, we have developed a concise synthetic route
to a â-(1f3)-D-rhamnotetraose, in which four challenging
â-glycosidic linkages are installed with a high degree of
stereoselectivity due to the disarming effect of the 4,6-O-
[1-cyano-2-(2-iodophenyl)-ethylidene] protecting group. The
key step is a late-stage radical deoxygenation, which occurs
simultaneously on all four residues of the tetramer.
1
Table 2. Comparison of H NMR Chemical Shifts and
Coupling Constants of Internal Residues of 21 with Those of the
Natural Rhamnan
chemical shift (ppm)
isolateda syntheticb
coupling constant (Hz)
H
isolateda
syntheticb
∆δ
H-1
H-2
H-3
H-4
H-5
H-6
4.81
4.26
3.87
3.54
3.46
1.34
4.66
4.11
3.72
3.38
complex
1.19
s
s
3.0
9.5
9.5
complex
6.0
-0.15
-0.15
-0.15
-0.16
2.7
9.7
9.2
5.9
-
Acknowledgment. We thank the NIH (GM 57335) for
financial support.
Supporting Information Available: Full experimental
details and characterization data. This material is available
free of charge via the Internet at http://pubs.acs.org.
-0.15
a Data for the isolated polysaccharide from ref 10. b Values for the
synthetic polysaccharide recorded in D2O at room temperature at 500 MHz.
OL061706M
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Org. Lett., Vol. 8, No. 19, 2006